Prolonged experimental stimulation of the rat thyroid induces two types of neoplasia, one follicular, and apparently of epithelial origin, and the other solid presumably arising from parafollicular cells. It is generally assumed that in the rat parafollicular cells represent embryonic ultimobranchial remains. In birds the thyroid gland apparently does not contain parafollicular cells since the ultimobranchial gland, in this species, is a separate organ. In the present study three question are being studied: (1) Does chronic iodine deficiency in the bird result in the formation of both types of tumors? (2) What effect does prolonged iodine deficiency and various levels of dietary calcium have on the thyroid of the rat, or of the bird? (3) What effect does alteration of calcium and iodine intake have on the ultimobranchial gland of the bird? Two methods of inducing thyroid tumors, (1) prolonged treatment with thiouracil, and (2) a low iodine diet with variable levels of dietary calcium are being used. At autopsy histological changes and function - as measured by radioiodine analysis - are being made.